1. Field of the Invention
The present invention relates to an apparatus and a method for lubricating an edge dam in a twin-roll type strip casting machine, in which strips are continuously manufactured by supplying a metal melt into between a pair of casting rolls. More specifically, the present invention relates to an apparatus and a method for lubricating an edge dam in a twin-roll type strip casting machine, in which a continuous uniform lubrication boundary layer is formed on the friction face between the casting rolls and the edge dam, so that the wear of the edge dam refractory material can be reduced, and that the friction faces of the casting roll can be protected, thereby improving the performance and the durability of the machine.
2. Description of the Prior Art
Generally, as shown in FIG. 1, a twin-roll type strip casting machine 100 supplies a melt 122 from a tundish 110 through a melt supply nozzle 112 into between a pair of revolving casting rolls 120, thereby directly manufacturing metal strips 130 of several mm.
Further, the casting rolls 120 are disposed in parallel with each other, and at both sides of the casting rolls, there are installed refractory plates called "edge dam 140". During the casting of the strips, the edge dams 140 prevent the spilling of the melt 122 from the casting rolls 120. In order to prevent the edge dams 140 from being pushed backward by the pressure of melt 122, hydraulic devices (not illustrated) support the back faces of the edge dams 140.
The edge dams 140 which prevent the spilling of the melt 122 are subjected to wear due to the frictions between the edge dams 140 and the revolving rolls 120. When the casting operation is carried out for a long time, the casting stability is aggravated due to an excessive wear of the edge dams 140. Therefore it is a necessity to reduce the wear rate so as to extend the life expectancy of the casting machine.
Further, the casting rolls 120 are also subjected to wear due to the frictions with the edge dams 140, with the result that the friction faces 125 of the casting rolls 120 are damaged. Such a damage of the casting rolls 120 accelerates the wearing of the refractory material 142 of the edge dams 140. Therefore, the friction faces 125 of the casting rolls 120 need to be protected from wear.
The edge dam 140 is made of a refractory base material which has a high thermal shock resistance, a superior high temperature strength, and a low thermal deformation, such as mullite, fused silica, silicon carbide or the like. In addition, the friction face of the edge dam 140 is added with a ceramic layer 142 which has a high thermal shock resistance, a high corrosion resistance (against the melt), a high toughness, a good machinability and the like, such as Si.sub.3 N.sub.4 -BN composite, AlN-BN composite, SiC-BN composite or the like (refer to FIG. 2). The ceramic layer 142 undergoes wear because it is closely contacted to the fast revolving casting rolls with a pressure of 2-4 Kg/cm.sup.2.
Generally, in the case where the composite material of the ceramic layer 142 is made to contain 40-60 vol % of BN, the wear rate is about 0.5 mm/min.
Therefore, if the casting operation is carried out for a long time, the wear becomes excessive, with the result that the casting stability is aggravated, and that the durability and life expectancy of the edge dam 140 are shortened.
If the hardness of the ceramic material of the ceramic layer 142 is high, although the wear rate is decreased, the friction face 125 of the casting roll 120 is damaged, giving a severe problem.
In this manner, there occur serious frictions between the ceramic layer 142 and the friction face 125 of the side of the casting rolls 120. If the hardness or the wear resistance of the ceramic layer 142 is higher than that of the friction face 125 of the casting roll 120, the wear of the edge dam 140 is small, but the wear of the friction face 125 of the casting roll 120 becomes high. In a worse case, pockmarks are formed on the casting rolls 120, with the result that the edge portions of the manufactured strips 130 become defective, and that the melt is spilled from between the casting rolls 120, these being a serious problem.
Therefore, Japanese Patent Gazette No. Hei-7-68352 proposes the following method. That is, a boron nitride composite ceramic layer which has a lubricating property and a proper wear resistance is attached on the portion of the edge dam 140 where frictions occur by being contacted with the casting roll 140. In this manner, the ceramic layer 142 of the edge dam 140, which has a low wear resistance in the relative terms, is made to be worn, thereby protecting the friction face 125 of the casting roll 120, and preventing the spilling of the melt.
However, the edge dams 140 are supported continuously by means of springs or hydraulic cylinders (not illustrated) to make the edge dams 140 closely contact with the casting rolls 120, and therefore, the wear of the ceramic layer 142 is increased too much after the elapse of casting time. Therefore, in the long run, the durability is aggravated owing to the increased wear.
Meanwhile, U.S. Pat. No. 5,201,362 discloses the following apparatus. That is, the closely contacting pressure of the edge dams 140 is controlled to a high level during the early casting stage, so that the wear of the ceramic layer 142 would be high in the early stage. However, when the casting is stabilized, the contacting pressure of the edge dams is lowered, and the contact force is controlled such that the wear rate should come into a proper range, thereby lowering the wear rate. In this method, after carrying out the casting for a long time, if the roughness of the friction face 125 of the casting rolls 120 becomes coarse, and if the physical properties of the ceramic material of the ceramic layer 142 are degraded at the high temperature, then the contacting pressure is lowered to obtain a decreased wear rate. In this manner, during the casting operation, an external force makes the edge dams 140 withdraw, and this withdrawing frequency is gradually increased. As a result, the quality of the edge portions of the metal strip 130 is degraded.
Meanwhile, in an attempt to overcome the damage of the casting rolls 120 and the excessive wear of the edge dams 140, Japanese Patent Laid-open No. Hei-9-108788 discloses the following apparatus. That is, a solid lubricant such as sintered boron nitride (BN) which is manufactured by a hot pressing process is contacted to the friction face 125 of the casting roll 120 so as for the solid lubricant to be clad on the friction face 125. The lubricant cladding layer acts as a lubricant on the friction face 125, thereby preventing the wear of the casting rolls 120 and the edge dams 140. However, if the surface of the friction face 125 is smooth (and not coarse), then the solid lubricant layer is hardly formed. On the other hand, if the surface of the friction face 125 is coarse, then the lubricant layer becomes too thick or the lubricating effect is decreased. Consequently, depending on the conditions of the surface of the friction face 125, either the lubricant layer is not formed, or a non-uniform lubricant layer is formed, this being a problem.